In the treatment of heart disease, one method of improving myocardial blood supply is called transmyocardial revascularization (TMR), the creation of channels in the myocardium of the heart. The procedure using needles in a form of surgical "myocardial acupuncture" has been used clinically since the 1960s. Deckelbaum. L. I., Cardiovascular Applications of Laser Technology, Lasers in Surgery and Medicine 15:315-341 (1994). The technique relieves ischemia by allowing blood to pass from the ventricle through the channels either directly into other vessels communicating with the channels or into myocardial sinusoids which connect to the myocardial microcirculation.
In the reptilian heart, perfusion occurs via communicating channels between the left ventricle and the coronary arteries. Frazier, O. H., Myocardial Revascularization with Laser--Preliminary Findings, Circulation, 1995; 92 suppl II!:II-58-II-65. There is evidence of these communicating channels in the developing human embryo. In the human heart, myocardial microanatomy involves the presence of myocardial sinusoids. These sinusoidal communications vary in size and structure, but represent a network of direct arterial-luminal, arterial-arterial, arterial-venous, and venous-luminal connections. This vascular mesh forms an important source of myocardial blood supply in reptiles but its role in humans is poorly understood.
Numerous surgical TMR studies have been performed, including early studies using needles to perform myocardial acupuncture, or boring, to mechanically displace and/or remove tissue. Such studies have involved surgically exposing the heart and sequentially inserting needles to form a number of channels through the epicardium, myocardium, and endocardium to allow blood from the ventricle to perfuse the channels. The early studies using needles showed that the newly created channels were subject to acute thrombosis followed by organization and fibrosis of clots resulting in channel closure. Interest in TMR using needles waned with the knowledge that such channels did not remain open. However, interest in TMR procedures has recurred with the advent of medical lasers used to create TMR channels. Histological evidence of patent, endothelium-lined tracts within laser-created channels shows that the lumen of laser channels can become hemocompatible and resists occlusion. A thin zone of charring occurs on the periphery of the laser-created channels through the well-known thermal effects of optical radiation on cardiovascular tissue. Additionally, recent histological evidence shows probable new vessel formation adjacent collagen occluded transmyocardial channels, thereby suggesting benefits from TMR with or without the formation of channels which remain patent.
Surgical TMR procedures using laser energy have been described in the prior art. U.S. Pat. No. 4,658,817 issued Apr. 21, 1987 to Hardy teaches a method and apparatus for surgical TMR using a CO.sub.2 laser connected to an articulated arm having a handpiece attached thereto. The handpiece emits laser energy from a single aperture and is moved around the surface of the heart to create the desired number of channels. U.S. Pat. No. 5,380,316 issued Jan. 10, 1995 to Aita et al. purports to teach the use of a flexible lasing apparatus which is inserted into the open chest cavity in a surgical procedure. A lens at the distal end of the flexible apparatus is used to focus laser energy, and the apparatus is moved about the surface of the heart to create the desired number of channels.
The foregoing discussion relates to surgical procedures, i.e. procedures which access the heart surgically, either via open heart surgery, or perhaps by minimally invasive surgical (MIS) methods if the design and size of the distal ends of the hand pieces are suitable for use in an MIS site. However, since TMR most often involves creating channels through the endocardium into the lower left chamber of the heart, it is desirable to create TMR channels in a percutaneous procedure, i.e. by extending a catheter apparatus through the vasculature into the ventricle and creating the channels through endocardial surfaces and into myocardium. Performing such percutaneous TMR is desirable for a number of reasons. Percutaneous catheter procedures are typically less traumatic to the patient compared to surgical procedures. Adhesions between the pericardial sac and epicardium are eliminated. Percutaneous TMR with a catheter apparatus also offers an alternative solution to persons who are not candidates for surgical procedures.
Because TMR procedures generally involve creating a plurality of channels within the myocardium, performing the procedure percutaneously requires the ability to steer a catheter apparatus through the vasculature and maneuver the apparatus within the ventricle of the beating heart as rapidly as possible to create the channels without subjecting the heart to the undue stress of a lengthy procedure. Additionally, the ability to control and stabilize the catheter apparatus against the beating heart wall while creating channels with a laser is desirable for percutaneous procedures to ensure creation of channels as desired and to ensure that the laser is fired only within the myocardial tissue. TMR channels should be spaced and grouped appropriately to achieve the desired result without weakening or rupturing the heart muscle.
The early myocardial acupuncture procedures were not performed percutaneously. The Hardy CO.sub.2 laser delivery system described above is rigid, relatively large, and not adaptable for percutaneous use. The Aita '316 patent does not suggest a method for percutaneous use of the single aperture, laser delivery device described therein for surgical use.
U.S. Pat. No. 5,389,096 issued Feb. 14, 1995 to Aita et al. purports to teach one method of percutaneous TMR using an elongated flexible lasing apparatus with control lines and a focusing lens structure at the distal tip. The method describes the use of pressure to attempt to stabilize the apparatus against the wall of the heart. The '096 apparatus requires movement and restabilization of the apparatus prior to the creation of each channel. Neither of these patents, nor any other prior art, describes or suggests creation of more than one TMR channels without the necessity for repositioning the catheter device.
Several prior art patents describe the use of catheters within the ventricle for percutaneous treatment of ventricular tachycardia. Such devices have a means to locate an arrhythmia site and ablate the site, at or just below the ventricle surface, using an electrode device or laser energy. U.S. Pat. No. 5,104,393 issued Apr. 14, 1992 to Isner teaches a catheter apparatus having a guiding Y-shaped sheath and guide catheter assembly for introducing an optical fiber into the ventricle. Positioning is described to enable a single burst of laser energy from a single aperture to ablate the site. However, positioning or specific steering means sufficient to create one or more TMR channels is not described or suggested.
U.S. Pat. No. 5,255,679 issued Oct. 26, 1993 and U.S. Pat. No. 5,465,717 issued Nov. 14, 1995 to, respectively, Imran and Imran et al., disclose non-laser, basket-shaped catheter apparatus for mapping and/or ablation of arrhythmia sites within the ventricle. A pull wire is used to expand the basket portion within the ventricle, and a plurality of electrodes on the arms of the basket are used for ablation. The basket device is designed to place the electrodes on the ventricle wall. Although the device allows for a fairly extensive mapping procedure without repositioning, no positioning means is provided for a laser delivery system to allow creation of TMR channels.
The use of superelastic and/or shape memory materials is widely known. Structure and Properties of Ti-NI Alloys: Nitinol Devices & Components, Duerig et al., In Press, Titanium Handbook, ASM (1994) In general, binary compositions of Nickel (Ni) and Titanium (Ti), yield alloys with shape memory and superelastic properties. These alloys are commonly referred to as Ni-Ti, nitinol, and other industry names. Their precise physical and other properties of interest are extremely sensitive to the precise Ni/Ti ratio used. Generally, alloys with 49.0 to 50.7 atomic % of Ti are commercially available, with superelastic alloys in the range of 49.0 to 49.4%, and shape memory alloys in the range of 49.7 to 50.7%. Due to a rapid decrease in the ductility of the material, binary alloys with less than 49.4 at % Ti are generally unstable. In general, these types of materials exhibit hysteresis, defined as a phenomenon exhibited by a system whose state depends on its previous history, and illustrated diagrammatically by the familiar upper and lower curves which meet at the ends and define an area under the curves. In the case of solid materials undergoing elastic hysteresis (as opposed to magnetic or electrical hysteresis), the curves are related to stress necessary to cause deformation or otherwise overcome existing stress in pre-stressed materials.
All properties of these materials change significantly as their respective "phase transformation temperatures" are approached. In general, at lower temperatures, these alloys will exist in a martensite state characterized as bard and easily deformed. However, in austenite, the high temperature phase, the alloys have a much higher yield and flow stresses. The addition of small amounts of third elements in the alloy can also have very significant effects on performance of the materials. Elements including but not limited to oxygen (O), nitrogen (N), iron (Fe), aluminum (Al), chromium (Cr), cobalt (Co) vanadium (V), zirconium (Zr) and copper (Cu), though having various effects on the Ni-Ti matrix, can have the tendency to increase strength, increase stiffness, control hysteresis and/or decrease or increase phase transition temperatures.
Ni-Ti products are commonly used in the form of cold drawn wire or as barstock. Tubing is also available. The toxicity of the alloy or the solubility or other compatibility with the biological environment in which catheter equipment is used is an important consideration. The alloys are commonly used in a cold worked and partially annealed condition. The partial anneal does not recrystallize the material but does bring about the onset of recovery processes. The extent of the post-cold worked recovery depends upon many aspects of the application, such as the desired stiffness, fatigue life, ductility, recovery stress, etc. Ni-Ti is difficult to join since most mating materials cannot tolerate the large strains experienced by Ni-Ti. Most connections will rely on crimped bonds. Although Ni-Ti can be brazed or welded to itself with relative ease, such as by resistance and with TIG methods, brazing or welding to other materials is difficult though proprietary methods do exist and are practiced in large volumes, for example in the production of eyeglass frames.
For the purposes of this disclosure, a distinction between superelastic materials and shape memory materials is made. Superelasticity refers to the highly exaggerated elasticity, or springback, observed in many Ni-Ti alloys deformed at a specific temperature. The function of the material in many of such cases is to store mechanical energy. Though limited to a rather small temperature range, these alloys can deliver over 15 times the elastic motion of a spring steel, i.e., withstand a force up to 15 times greater without permanent deformation. Shape memory materials will refer to those materials which can be deformed, but which will freely recover their original shapes during heating, often utilizing electrical resistivity, or which will develop a large recovery stress when recovery is prevented. Applications of these shape materials include various types of fasteners and tube/pipe connectors. It will be understood that a significant difference or distinction between such superelastic and shape memory materials is the phase or transition temperature below which they may be deformable and above which they will return to their original, preformed shape. With regard to the present invention, it will be understood that the transition temperature of materials must, in general, be somewhat above body temperature by at least 10.degree.-12.degree. C.
U.S. Pat. No. 3,890,977 issued Jun. 24, 1975 to Wilson teaches kinetic memory electrodes, catheters and cannulae. These devices incorporate a material, such as a Ni-Ti alloy, having heat-activated mechanical memory properties. The device is formed into an operative shape at a high temperature. Then, at a low temperature below its transitional temperature, it is reformed into a shape for ease of insertion into a guide catheter or the like or otherwise through a portion of a patient's vasculature or other body lumen. When located in the organ or other desired region, those portions of the device constructed using such shape memory materials are heated to above their transitional temperatures, using electrically resistive elements, thereby returning the catheter to its original annealed anchoring or proper locating shape. An important drawback of the Wilson apparatus is that heat must be applied to the catheter tip. Complicated construction and electrical power distribution must be considered.
As can be seen from a description of the prior art above, percutaneous TMR catheters are virtually unknown with the exception of the catheter briefly described in the '096 Aita patent. There is a need in the art for a percutaneous TMR catheter shaped to correspond to the contours of the ventricle, having means for easily positioning and repositioning the catheter against the ventricle wall, and having a port for a laser delivery means to enable rapid creation of a plurality of appropriately grouped and spaced TMR channels without repositioning the catheter. Providing a catheter made of a superelastic material to avoid the heating or electrical requirements of shape memory materials would be particularly desirable.